Plug in the drive. Go to Macintosh HD, then go to Applications folder inside that is another folder called Utilities inside that is an application called Disk Utility. Double click on that and when it opens you will see your hard drive on the left. Highlight your hard drive and click on Verify Disk on the right. If Verify Disk is not OK try to Repair.

Please go to the Apple logo on the upper left of the screen, go to About This Mac, and tell me the System Version, the Processor and the Memory.

Go to Macintosh HD, then go to Applications folder inside that is another folder called Utilities inside that is an application called Disk Utility. Double click on that and when it opens you will see your hard drive on the left. Highlight your hard drive and click on Verify Disk on the right. If that is OK click on Repair Permissions. If Verify Disk is not OK try to Repair.

Put the system disk in and restart holding down the C key until you see the Apple logo. When you get to the install screen DO NOT INSTALL, you will see at the top of the screen a Utilities menu drop down. Go the Disk Utility, click on your drive on the left and click on verify disk. If that is Ok, Repair Permissions. If not OK try to Repair Disk.